Not applicable.
Not applicable.
(1) Field of the Invention
The present invention generally relates to overmolded circuit boards with surface-mount (SM) devices, such as flip chips. More particularly, this invention relates to an overmolded circuit board and process by which the circuit board is overmolded and a surface-mount device on the circuit board is simultaneously underfilled with the overmold material, without the creation of voids between the device and circuit board.
(2) Description of the Related Art
Circuit boards with semiconductor devices such as flip chips must often be protected from the environment in which the board is employed. One widely-practiced approach is to enclose a circuit board in an assembly composed of a pair of case halves that are secured together with fasteners to form an enclosure. Because this assembly process is labor intensive, less complicated assembly processes have been sought. One solution is an overmolded assembly disclosed in commonly-assigned U.S. Pat. No. 6,180,045 to Brandenburg et al. This assembly includes a circuit board, a heatsink with pedestals that thermally contact one or more circuit components mounted to the circuit board, and an optional retainer that biases the components into contact with the heatsink pedestals. An overmolded body encases the circuit board and retainer such that, with the heatsink, the overmolded body forms a protective environmental seal around the circuit board and its circuit components.
Underfilling is well known for promoting the reliability of flip chips and ball grid array (BGA) packages attached to organic circuit boards with numerous solder bumps connections, which result in a gap being present between the component and circuit board. Filling this gap, also termed the xe2x80x9cstand-offxe2x80x9d height of the device, with an appropriate material has been shown to greatly improve the thermal cycle life of the solder connections. However, for optimum reliability underfilling must completely fill the gap, and therefore has typically required the use of a low-viscosity underfill material, such as a specially-formulated thermosetting epoxy. In a conventional underfill process, the underfill material is placed at the perimeter of the component so that capillary action draws the material beneath the component. To avoid the entrapment of air that would form voids in the underfill material beneath the component, the underfill material is typically placed along only one or two sides of the component, such that as the underfill material flows through the component-to-circuit board gap, air is pushed out ahead of the underfill material.
A second underfill method known in the industry is primarily used for packaged flip chips, such as BGA packages, and combines the overmolding and underfilling operations. However, this method has conventionally been limited to overmolding only one side of a circuit board. The circuit board is placed in a mold that defines a sufficiently restrictive gap between the component and mold surface, so that as the molding material is injected at one end of the mold cavity, the molding material is forced into the gap between the component and circuit board at a rate that prevents air from being entrapped in the gap. Difficulties arise when attempting to overmold both surfaces of a circuit board and simultaneously underfill devices on the board. As represented in FIG. 1, with finer solder bump pitches (corresponding to a low stand-off height) there is a greater propensity for a large underfill void 110 to form between a chip 114 and its circuit board 112 during the overmold process. FIG. 1 shows the result of a molding material 116 flowing faster through larger openings in the mold cavity (e.g., the gap between the chip 114 and the upper mold 118) than through the smaller (e.g., 0.003 inch (about 75 micrometers)) gap between the chip 114 and board 112. Once the entire periphery of the chip 114 is covered with the molding material 116 from above, air is trapped beneath the chip 114 to produce the void 110.
A common method to reduce void formation in various molding operations is to use one or more vent holes to avoid air entrapment. However, as represented in FIG. 2, when an attempt was made to apply the use of a vent 120 to a circuit board assembly of the type shown in FIG. 1, the result was not the elimination of voids, but only the shifting of the void 110 to another region beneath the chip 114. The cause of this shift was that each chip 114 impedes the flow of underfill material 116, resulting in uneven flow of underfill material 116 through the cavities on opposite sides of the circuit board 112. Underfill material 116 entered the gap between the lower chip 114 and the circuit board 112 before all of the air was purged from the gap, entrapping air between the vent 120 and the edge of the chip 114 facing the flow of material 116. Accordingly, the vent 120 did not enable complete underfilling of the chips 114 on the circuit board 112 while simultaneously overmolding both surfaces of the board 112.
According to the present invention, there is provided an overmolded circuit board assembly and a method for forming the assembly. The assembly and method entail overmolding both surfaces of a circuit board and underfilling one or more surface-mount circuit devices attached to at least a first surface of the board with solder bump connections, with the result that the circuit device is spaced above the first surface of the circuit board by the solder bump connections so as to define a gap therebetween. The invention entails the use of a cavity or pocket defined in the first surface of the circuit board beneath the circuit device, such that the cavity communicates with the gap between the circuit board and the circuit device, but is closed off from the surface of the circuit board opposite the device. As a result of the presence of the cavity, gas (e.g., air) that is trapped in the gap by a molding material and would otherwise form a void beneath the device is compressed within the cavity as the molding material completely fills the gap between the circuit board and the circuit device and encapsulates the circuit device and both surfaces of the circuit board.
From the above, it can be seen that the present invention enables air trapped under a circuit device (e.g., a flip chip) during a simultaneous overmolding and underfilling process to escape to a non-critical area in or under the circuit board. More particularly, the trapped air is forced into the cavity in the circuit board surface so as not to contact the flip chip. Since circuit boards are typically much thicker than the gap between the board and devices mounted to the board, the cavity can be much smaller in diameter than the diameter of a void that would be formed by the trapped air that the cavity is intended to accommodate. As such, a single cavity can often be sized to completely eliminate voids beneath an underfilled flip chip.
The ability to eliminate underfill voids with this invention serves to significantly improve the reliability of the solder connections that attach flip chips or other SM circuit devices when subjected to moisture and thermal cycling. This benefit of the invention is achieved while simultaneously overmolding the entire circuit board assembly to produce a protective environmental seal, and with the use of existing materials with little or no changes in the manufacturing process.
Other objects and advantages of this invention will be better appreciated from the following detailed description.